This invention relates to a device and method for infrared-optical determination of the concentration of at least one chemical analyte in a liquid sample, where the liquid sample is passed through a measuring cell and irradiated with infrared radiation of a single narrow waverange and infrared absorption is measured.
Measuring substance concentrations is required in many fields of science and technology, such as chemistry, process-, manufacturing-, or medical engineering, analytical chemistry in ecology or food technology. For many analytes to be determined the infrared region of the spectrum offers characteristic absorption bands from whose intensities the analyte concentration may be determined in principle. Infrared spectrometers are expensive and complex devices, however, which are awkward to handle and ill-suited for on-line measurement in a production plant, for example. Moreover, the analyte is often provided at low concentration in a solvent which itself will absorb in the respective waverange. In aqueous solutions above all, the absorption of water will produce a dominant background signal in the entire infrared region. As a consequence, direct determination of the concentration will be difficult and inaccurate.
This problem may be solved by taking measurements at two wavelengths at least, one of which is chosen such that only the background absorption by the solvent is detected, whereas the other wavelength is within the absorption band of the analyte, thus covering analyte absorption and background absorption. The two wavelengths may be generated by separate radiation sources, or from the spectrum of a single broadband source by means of two band filters. The disadvantage of this method, however, is that the ratio of the intensities of the two wavelengths must be absolutely constant to prevent measuring errors. When two separate light sources are used, intensities will fluctuate independently of each other and a constant intensity ratio will be difficult to obtain, necessitating complex provisions for control. Even if a single source is used the intensity ratio will change along with changes in temperature and in the emission properties of the radiation source as well as the transmission properties of the band filters. Such a change again may be compensated only by monitoring and readjusting both intensities, which will entail considerable technical effort and financial expense.